Barium titanate has been widely employed as a functional material in, among others, dielectric materials, multi-layer ceramic capacitors, and piezoelectric materials. Electronic parts of small size and light weight have been developed, and in accordance with this trend, demand has arisen for development of a process for producing barium titanate having smaller particle size and exhibiting excellent electric characteristics, such as high dielectric constant.
Barium titanate having high tetragonality content is known to have high dielectric constant, but so far, attempts to reduce the particle size of such barium titanate to a desired level have failed, whereas in the case of barium titanate having small particle size, the tetragonality content thereof has failed to be increased, and thus the dielectric constant of such barium titanate cannot be increased satisfactorily.
Examples of processes for producing titanium-containing composite oxide particles such as barium titanate include a solid-phase process in which powders of an oxide and a carbonate, serving as raw materials, are mixed in, for example, a ball mill, and the resultant mixture is allowed to react at a temperature as high as about 800° C. or higher, to thereby produce a product; an oxalate process in which an oxalic acid composite salt is prepared, and the composite salt is thermally decomposed, to thereby produce titanium-containing composite oxide particles; an alkoxide process in which a metal alkoxide serving as a raw material is subjected to hydrolysis, to thereby yield a precursor; a hydrothermal synthesis process in which a raw material is allowed to react in an aqueous solvent at high temperature and high pressure, to thereby yield a precursor; a process in which a product obtained through hydrolysis of a titanium compound is reacted with a water-soluble barium salt in a strong alkaline aqueous solution (Japanese Patent No. 1841875); and a process in which a titanium oxide sol is reacted with a barium compound in an alkaline aqueous solution (International Patent Publication WO00/35811).
Although the solid-phase process attains low production cost, titanium-containing composite oxide particles produced through the process have a large particle size, and the particles are unsuitable for use as a functional material such as a dielectric material or a piezoelectric material. When the thus-produced particles are subjected to grinding to thereby form fine particles, strain occurs in the resultant fine particles. Therefore, barium titanate having high tetragonality content; i.e., barium titanate having high dielectric constant, fails to be produced.
The oxalate process enables production of particles having a particle size smaller than that of particles produced through the solid-phase process. However, particles produced through the oxalate process contain a carbonate group derived from oxalic acid. Therefore, the oxalate process cannot produce barium titanate exhibiting excellent electric characteristics.
The alkoxide process and the hydrothermal synthesis process enable production of barium titanate having a very small particle size. However, the thus-produced barium titanate contains a large amount of a hydroxyl group derived from water. Therefore, the barium titanate fails to exhibit excellent electric characteristics. Barium titanate produced through the alkoxide process contains a carbonate group.
Since the hydrothermal synthesis process is carried out at high temperature and high pressure, the process requires exclusive equipment, and thus production cost increases.
The processes disclosed in Japanese Patent No. 1841875 and International Patent Publication WO00/35811 employ potassium hydroxide or sodium hydroxide as an alkali. Therefore, the processes require a step of removing such an alkali after completion of reaction. Since barium elutes and a hydroxyl group enters the resultant barium titanate during the removal step, the barium titanate fails to have high tetragonality content.
The present invention contemplates provision of a barium titanate having a small particle size, containing small amounts of unwanted impurities, and exhibiting excellent electric characteristics, which can be employed for forming a dielectric ceramic thin film required for a small-sized capacitor which enables production of a small-sized electronic apparatus; and a process for producing the barium titanate.